1. Field of the Invention
The present invention relates to a mill inert apparatus for a coal pulverizer in which an inert medium is introduced into a coal pulverizer for pulverizing coal to lower an oxygen concentration in the coal pulverizer, whereby the explosion of the pulverized coal is prevented, and it also relates to a method for preventing the explosion.
2. Description of the Related Art
In recent years, there has been a tendency to use coal containing a large amount of ignitable and volatile components in pulverized coal-fired boilers. If such an ignitable coal is pulverized in a coal pulverizer, the explosion of the pulverized coal is particularly liable to occur. In order to prevent this explosion, a mill inert apparatus and a mill inert method which are used in an inert medium, such as N.sub.2, C0.sub.2 or a vapor, is introduced into the coal pulverizer for lowering the oxygen concentration in the coal pulverizer.
FIG. 4 shows the construction of this kind of mill inert apparatus. Reference numeral 1 is a coal feeding device for feeding coal, and numeral 2 is a coal pulverizer for pulverizing the coal fed from the coal feeding device 1. The coal, which has been pulverized to a predetermined particle size in the coal pulverizer 2, is dried by hot air introduced through a hot air duct 3, and is then forwarded to a burner 10.
Also, numeral 11 is a vapor line extending from a boiler, and a vapor led to a branch from this vapor line 11 is used as the inert medium. The inert medium led into the branch is forwarded via a pressure-reducing valve 4 and a temperature-lowering device 5 to reduce the pressure and to lower the temperature of the inert medium, and the thus regulated inert medium is then introduced into the coal pulverizer 2 through the above-mentioned hot air duct 3.
The amount of the inert vapor introduced into the coal pulverizer 2 through the hot air duct 3 is controlled by an orifice 12 disposed on the downstream side of the temperature-lowering device 5.
Furthermore, the amount of the hot air introduced into the coal pulverizer 2 through the hot air duct 3 is metered by an orifice 13 disposed on the hot air duct 3, and then controlled by a damper 14 disposed on the upstream side of the orifice 13 in the hot air duct 3 in accordance with a metered result.
This mill inert apparatus has the drawback that when the inert vapor led from the vapor line 11 is forwarded via the pressure-reducing valve 4 and the temperature-lowering device 5 to reduce the pressure and to lower the temperature of the inert medium, the vapor reaches a saturated state and generates a liquid which herein will be called drain. If directly introduced into the coal pulverizer 2, the drain has a bad influence on cast parts in the coal pulverizer 2, so that these cast parts may be damaged.
Moreover, in the above-mentioned conventional mill inert method, the desired oxygen concentration obtained by dilution with the inert medium is set to a range of from 7 to 11%. However, since this oxygen concentration level is too low, a large amount of the inert medium is consumed, so that annexed facilities are excessively required. In addition, the combustion of the burner may become unstable, and there is a risk that fire extinction or the like occurs.
Furthermore, in the conventional technique, a CO concentration in the coal pulverizer is detected by means of a CO meter, and when the CO concentration exceeds a predetermined value, the inert medium is fed. Therefore, the CO meter tends to be worn and clogged with the inert medium to diminish the reliability of the CO meter.